I wondered how this would vary with clock speed - and there is always a trade-off with more speed needing more power...
Using a PIC16F887 - the internal clock can be set at speeds up to 8MHz - but an external clock can be up to 20MHz,
I wrote a simple blinkie - and connected a pin to a logic analyser (with a 1ms delay in the loop)
In the chip options I set clock to XC - and I connected a function generator to pin OSC1 (and ground) I added a capacitor between OSC2 and ground - but this didn't seem to be necessary. I also set clock speed as 20MHz
I set the function generator to a sine wave at 20MHz and on turning the signal on got a neat set of pulses output, I could reduce the clock speed (and even overclock 24MHz worked for a handy 20% speed boost

Setting the clock to 10Mhz - gave a 2ms pulse as expected - and the chip was happy at alll the clock speeds I tested (down to 32kHz)
I also tested a PIC12F1840 - and the best power savings are to be had by using sleep wherever possible, a low speed clock and reducing the voltage - and the PIC was using about 300nA when sleeping ! For battery applications this is good - however other times what is needed is speed and power consumption is less of a concern...
As most (many?) oscilloscopes have a signal generator - this can be fairly easy to implement and use...
Martin